The present invention relates to a control device of a gasoline direct-injection engine.
Conventionally, gasoline direct-injection engines for retarding an ignition timing of fuel have been known.
For example, JP2013-057268A discloses an engine. In order to reduce a pressure increase rate within a cylinder in combustion (a rate of an in-cylinder pressure change with respect to a crank angle change), the engine controls a combustion timing so that a timing at which the pressure increase rate during motoring of the engine becomes a largest, i.e., most negative value (lowest rate) is in a combustion period. Specifically, the pressure increase rate during the motoring reaches its highest value slightly before a compression top dead center (CTDC), becomes zero at the CTDC, then becomes a negative value after the CTDC, and eventually becomes the largest negative value. In other words, the engine retards an ignition timing to a predetermined timing in the expansion stroke to overlap the combustion period with the timing at which the pressure increase rate during the motoring becomes the largest negative value. Thus, the pressure increase rate in the combustion is reduced and a vibration noise (NVH) level is reduced.
Note that, the phrase “during motoring of the engine” used herein indicates a state of the engine while a crankshaft of the engine is rotated by an electric motor and which is not accompanied by a combustion. For a purpose of research, for example, a comparison of the pressure increase rate within the cylinder based on whether the combustion is caused or not, is performed under this state.
However, since an in-cylinder temperature decreases as the expansion stroke progresses, if the ignition timing is retarded excessively, there is a risk of causing a misfire. Particularly, since a decreasing speed of the in-cylinder temperature in the expansion stroke is higher as the compression ratio of the engine is higher, the ignition timing cannot be retarded much.